According to Japanese Patent Application Publication No. JP-A-6-297985, a rollover prevention control is disclosed. A vehicle's center of gravity position is preset based upon the vehicle's weight or the like. If the center of gravity position varies according to movement of the vehicle, a rollover prevention control is executed in response to such changes. More specifically, a roll angle sensor and a displacement-from-ground sensor are used to calculate the roll angle and a change amount in the center of gravity height resulting from vehicle movement. Based upon the calculated roll angle and the change amount in the center of gravity height, a start threshold value for the rollover prevention control is changed.
However, as in the case of sport utility vehicles (SUVs) and trucks, that is, vehicles loaded with a wide variety of cargo, a center of gravity position and weight with cargo will change considerably depending on the cargo arrangement and method. For example, the center of gravity height is high for a truck loaded up to carrying capacity. Even though the vehicle weight is the same, the center of gravity position in an SUV with more people sitting on right side seats is different from that in the SUV with an equal number of people sitting on the right and left sides.
For this reason, a method such as described in Japanese Patent Application Publication No. JP-A-6-297985 is inadequate. Namely, the method in Japanese Patent Application Publication No. JP-A-6-297985 sets the center of gravity position for each vehicle based upon the vehicle weight or the like. Since this is used for compensating a control start threshold value considering a vary amount of the center of gravity due to vehicle movement, the compensation range is small. Therefore, extensive changes in the vehicle weight or the like will result in a failure to adapt to cases in which center of gravity positions are completely different despite being for the same vehicle.
On the other hand, a method in Japanese Patent Application Publication No. JP-A-11-83534, proposes calculating a center of gravity height corresponding to changes in the vehicle cargo amount from a roll angle, the vehicle weight, and a lateral acceleration (hereinafter referred to as a “lateral G”). Thereafter, a start threshold value for an accompanying rollover prevention control is thus changed.
However, the three parameters consisting of the roll angle, the vehicle weight, and the lateral G are difficult to measure precisely. Therefore, precisely calculating the center of gravity height is difficult when calculated using the three parameters.
With regard to the roll angle, for example, the vehicle tilts when the effects of a right and left cant of a road surface affects, when more people sit on either the right or left side seats or when packages are loaded such that either the right or left side of the vehicle is heavier than the other. For this reason, it is difficult to determine where to set the zero position, i.e., the zero point, of the roll angle, which makes it impossible to obtain a precise roll angle. Furthermore, in Japanese Patent Application Publication No. JP-A-11-83534, the roll angle is calculated from an integral value of a roll rate. However, use of such an integral method accumulates integral errors, which lower the precision of the obtained roll angle.
With regard to the vehicle weight, an equation F=MG (F: brake force acting on vehicle, M: vehicle mass, G: vehicle deceleration) is used to calculate a vehicle weight M from a brake force F, which is calculated from a vehicle braking pressure P, and a vehicle deceleration G detected by an acceleration sensor.
However, the relationship between the vehicle braking pressure P and the vehicle deceleration G changes due to the longitudinal gradient of the road surface (uphill and downhill), air resistance, property changes caused by heat generated in brake materials, hysteresis, and the zero point setting for a longitudinal G sensor. The effect of such change is particularly large in a soft braking region often used during normal brake use. In addition, at times when an ABS control or the like intervenes during sudden braking, the relationship between the vehicle braking pressure P and the vehicle deceleration G is such that F≠MG. Therefore, making a precise calculation of the vehicle weight is difficult.
With regard to the lateral G, if the vehicle is tilted as shown in FIG. 5, a gravity component becomes included in an output signal of the lateral G sensor. In reality, the lateral G (hereinafter referred to as an “actual lateral G”) obtained from the detection signal of the lateral G sensor is not the ideal lateral G, which should indicate only a horizontal-direction component.